This invention relates generally to subcritical vapor compression systems that use thermostatic expansion valve devices to control compressor suction superheat and, more particularly, to a method and apparatus for determining refrigerant charge adequacy in an air conditioning system.
Maintaining proper refrigerant charge level is essential to the reliable and efficient operation of an air conditioning system. Improper charge level, whether in deficit or in excess, can cause premature compressor failure and diminish system efficiency. An over-charge in the system results in compressor flooding with liquid refrigerant, and/or excessive discharge pressure, which, in turn, may be damaging to the motor and mechanical components. Inadequate refrigerant charge can lead to diminished system capacity and efficiency and potential compressor damage. Low charge also causes reduced refrigerant mass flow, decreased refrigerant pressure entering the compressor and/or an increase in refrigerant temperature entering and leaving the compressor, all of which may cause thermal over-load of the compressor. Thermal over-load of the compressor can cause degradation of the motor winding insulation, thereby bringing about premature motor failure. Additional effects of low refrigerant charge can include diminished or total loss of lubrication of mechanical components, which can lead to catastrophic compressor failure.
Charge adequacy has traditionally been checked manually by trained service technicians using pressure measurements, temperature measurements and a pressure to refrigerant temperature relationship chart for the particular refrigerant resident in the system. For refrigerant vapor compression systems which use a thermal expansion valve (TXV), the superheat of the refrigerant entering the compressor is normally regulated at a fixed value, while the amount of subcooling of the refrigerant exiting the condenser may vary. Even so, in such systems, the “subcooling method” is customarily used as an indicator for charge level. In this method, the amount of subcooling, defined as the saturated refrigerant temperature at the refrigerant pressure at the outlet of the condenser coil for the refrigerant in use, a.k.a. the refrigerant condensing temperature, minus the actual refrigerant temperature measured at the outlet of the condenser coil, is determined and compared to the manufacturer's published subcooling value for the particular air conditioning or heat pump system. Generally, an acceptable subcooling value for a subcritical refrigerant vapor compression system operating as a residential or light commercial air conditioner lies between 10 and 15° F.
Typically, the technician uses a pressure gauge to measure the refrigerant pressure at the condenser outlet and a temperature gauge to measure the refrigerant line temperature at a point downstream with respect to refrigerant flow of the condenser coil and upstream with respect to refrigerant flow of the expansion valve, generally near the exit of the condenser. With these refrigerant pressure and temperature measurements, the technician then refers to the pressure to temperature relationship chart for the refrigerant in use to determine the saturated refrigerant temperature at the measured pressure and calculates the amount of cooling actually present at the current operating conditions, that is outdoor temperature, indoor temperature, humidity, indoor airflow and the like. If the measured amount of cooling lies within the range of acceptable levels specified by the manufacturer at corresponding ambient and operating conditions, the technician considers the system properly charged. If not, the technician will adjust the refrigerant charge by either adding a quantity of refrigerant to the system or draining a quantity of refrigerant from the system, as appropriate. Methods for determining the refrigerant charge level in an air conditioning system are disclosed in U.S. Pat. Nos. 5,239,865; 5,987,903; 6,101,820; and 6,571,566.
As operating conditions may vary widely from day to day, the particular amount of cooling measured by the field service technician at any given time may not be truly reflective of the amount of subcooling present during “normal” operation of the system. Thus, this charging procedure is also an empirical, time-consuming, and a trial-and-error process subject to human error. Therefore, the technician may charge the system with an amount of refrigerant that is not the optimal amount charge for “normal” operating conditions, but rather with an amount of refrigerant that is merely within an acceptable tolerance of the optimal amount of charge under the operating conditions at the time the system is charged. This results in human error added to the charging of the system with refrigerant. Therefore, it is desirable to provide a method and device for automatically indicating the status of the refrigerant charge within an operating system over a wide range of actual operating conditions. It is also desirable to provide a visual interface in association with such a device to indicate whether or not the system is properly charged.